The flux used for soldering generally has efficacy to chemically remove any metal oxides existed on a solder alloy and a metal surface of a join object which is subject to the soldering and to allow any metallic elements to move through a boundary therebetween. Therefore, the soldering using the flux enables intermetallic compounds to be formed between the solder alloy and the metallic surface of the join object, thereby attaining any strong joining.
On the other hand, components of the flux include any components which cannot decompose or evaporate by heating during the soldering and they remain around the soldered parts as flux residue after the soldering.
Meanwhile, together with recent development in miniaturization of electronic components, an electrode to be soldered in the electronic component also becomes miniaturized. Therefore, an area which can be soldered by the solder alloy is limited so that it may be impossible to get sufficient joining strength by only the solder alloy.
Accordingly, a technology has been proposed in which the electronic component and the like are adhered by covering the join object part to be soldered with any resins such as underfill as parts-adhering means for strengthening the joining of the soldering.
Here, when the flux residue remains around the join object part to be soldered, the flux residue hinders adhering the join object part to the resin, so that it is impossible to maintain the strength thereof. Therefore, in order to cover the join object part by the resin, it is required to clean the flux residue. It, however, takes any times and costs to clean the flux residue.
Accordingly, a technology has been proposed in which a thermosetting resin contained in the flux can maintain its no-cured state after reflowing so that the resin contained in the flux residue can be compatible with the underfill (See, for example, Patent Document 1). In addition, a technology has also been proposed in which thermoplastic property is given to the resin contained in the flux and the flux contained in the flux residue is fluidized when the underfill is applied so as to be compatible with the underfill (See, for example, Patent Document 2).